This invention relates to a primary battery in which a light metal such as lithium is used as a negative active material, especially a primary battery, the exchange timing of which is easily recognized.
Recently, there has been increased use of a battery having high energy density for an electronic apparatus, such as arm watch, small typed calculator, etc., in which a material for indication such as liquid crystal, light emitting diode, etc., is used.
As an example of the battery having high energy density, primary batteries have been developed in which light metals such as lithium, sodium, etc., are used as negative active materials.
Those batteries above have the advantage that the battery voltage does not change very much with the time duration of use, but have the simultaneous disadvantage that the battery voltage decreases suddenly when the life time of the battery comes to an end, which results in the decrease or stop of the function of the apparatus using those type of the battery.
Those problems seem to be solved by using a mechanism for indicating life time such as lamp, volt meter, etc., along with the battery; but, it is troublesome to watch the volt meter all the time and difficult to define the voltage value for causing the mechanism to indicate life time to operate due to the sudden drop of the battery voltage.
One solution for those problems above is provided by changing the characteristics of the battery so that the discharge voltage of the battery changes more than two steps, whereby the life time of the battery is easily indicated (Japanese Patent Laid-Open No. 51-62333).
Specifically speaking, the Japanese Patent Laid-Open No. 51-62333 discloses a battery comprising a negative active material, a positive active material and a non-aqueous electrolyte, wherein the positive active material comprises a main positive active material which takes part only in a primary discharge and a sub-positive active material which takes part only in a secondary discharge.
The solution above has advantage that the life time of the battery is indicated through the difference of the discharge voltage, but has a simultaneous disadvantage that the energy density on which the life time of the battery depends is defined by the amount of the main positive active material taking part only in the primary discharge. This is because the life time of the battery is defined by the time duration of the primary discharge.
Accordingly, as the amount of the sub-positive active material contained in the battery increases, the amount of the main positive active material decreases and the life time of the battery decreases, because the total volume of the battery is limited.